A comparative investigation was conducted on wettable and non-wettable glass-fiber filters to clarify the mechanism(s) leading to flow induced entrainment of oil and to determine the influence of oil concentration and filtration velocity on the rate of secondary droplet production. Entrainment rates from a wet- table and a non-wettable glass-fiber filter were measured over prolonged periods of time and with high time resolution by two optical techniques covering the drop size regions of about 0.5–10 µm and 150– 2000 µm. The rates were compared with oil drainage patterns recorded over time, and with estimates of the forces required to detach drops by blow-off as compared to drop run-off (drainage) or to form air bubbles in a drainage film.
It was found that initially oleophobic (i.e. non-wettable) media underwent a one-time transition in apparent wettability during normal operation, due to a change in drainage behavior from individual drops to a film-like flow pattern of oil. The transition was associated with a temporary increase (‘‘burst”) in entrainment rate. After that, both types of media had steady-state entrainment rates that were com- parable within a factor of two and depended mostly on the loading rate of the filter (which in turn deter- mines the drainage rate), but showed little or no at all on filtration velocity. In conclusion, blow-off was ruled out as a mechanism contributing significantly to oil entrainment under the prevailing operating conditions, regardless of media wettability. Secondary droplets are due to fragments from bursting air bubbles, which are formed within the liquid film on the downstream face of the media. The entrainment burst of an initially non-wettable medium is also due to bubbles formed on draining oil drops.

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